1. Field of the Invention
The present invention relates to a wireless communication technique. More particularly, the invention relates to wireless communication that utilizes an STBC (Space-Time Block Coding) transmission scheme and a non-STBC transmission scheme, etc.
2. Description of the Related Art
Conventionally, wireless LAN systems use an ISM band such as IEEE 802.11b and IEEE 802.11g. This frequency band is one that can be used freely by receiving wireless approval but, for this reason, the number of terminals used increases and this leads to congestion. It is necessary to raise the efficiency of frequency utilization in order to broaden the band of the wireless communication system used within this frequency band.
With this as a background, the IEEE 802.11n standard exists for the purpose of broadening the frequency band of a wireless communication system. The IEEE 802.11n standard is optimum for raising the efficiency of frequency utilization. The IEEE 802.11n standard utilizes a MIMO (Multiple Input, Multiple Output) technique, which employs a plurality of transmit antennas and a plurality of receive antennas. Further, the technique referred to as STBC (Space-Time Block Coding) transmission also is utilized. STBC transmission forms MIMO channels and, on the transmit side, subjects a single information signal sequence to space-time coding in the time and space directions and transmits the coded signal sequence from a plurality of transmit antennas in parallel. On the receive side, the STBC transmission scheme performs decoding using a transfer function of each estimated channel. As a result, an effect equivalent to the transmission diversity effect can be obtained.
Since the STBC transmission scheme sends the same transmit data from a plurality of antennas in parallel, the transmission rate is low. However, the receive CNR (carrier-to-noise ratio) can be improved by the transmit diversity effect and highly reliable transmission is possible.
Further, another method of improving frequency utilization efficiency is non-STBC transmission. This scheme similarly forms MIMO channels using a plurality of transmit antennas and a plurality of receive antennas. On the receive side, this scheme decodes transmit signals from each of the transmit antennas using the transfer functions of the MIMO channels estimated from the receive signals of the plurality of receive antennas. This method uses a plurality of transceive antennas, thereby increasing the number of spatially independent transmission channels by the number of transmit antennas. By transmitting items of independent data through each of the transmission channels, the transmission rate is increased.
The non-STBC transmission scheme uses a plurality of antennas and forms a unique stream by executing calibration processing, thereby realizing high-speed wireless communication. Since the unique stream is dependent upon the wireless propagation characteristic, the non-STBC transmission scheme can no longer be maintained and performing high-speed wireless communication becomes difficult if the wireless propagation characteristic changes.
The STBC transmission scheme is one that has an effect equivalent to transmit diversity. It is a highly reliable transmission scheme in which, even with a single antenna on the receive side, a reception characteristic equal to that in a case where reception is performed by two antennas is obtained.
FIG. 9 is a diagram useful in describing the concept of a communication system that uses a plurality of antennas.
In FIG. 9, a base station device 901 has antennas 903 and 904 and communicates via terminal device 902 and wireless link. The terminal device 902 has antennas 905 and 906.
Let h11(t) and h12(t) represent the channel transfer function of the transmission channel between the antennas 903 and 905 and the channel transfer function of the transmission channel between the antennas 903 and 906, respectively. Similarly, let h21(t) and h22(t) represent the channel transfer function of the transmission channel between the antennas 904 and 905 and the channel transfer function of the transmission channel between the antennas 904 and 906, respectively. The transfer functions h11(t), h12(t), h21(t) and h22(t) are functions which the terminal device 902 estimates using a propagation environment estimation symbol, such as a pilot signal, transmitted from the base station device 901.
With the STBC transmission scheme, the following equation holds:
                              (                                                                      R                  ⁢                                                                          ⁢                  1                  ⁢                                      (                    i                    )                                                                                                                        R                  ⁢                                                                          ⁢                  1                  ⁢                                      (                                          i                      +                      1                                        )                                                                                )                =                              (                                                                                h                    ⁢                                                                                  ⁢                    11                    ⁢                                          (                      i                      )                                                                                                            h                    ⁢                                                                                  ⁢                    21                    ⁢                                          (                      i                      )                                                                                                                                        h                    ⁢                                                                                  ⁢                    21                    *                                          (                                              i                        +                        1                                            )                                                                                                                                  -                      h                                        ⁢                                                                                  ⁢                    11                    *                                          (                                              i                        +                        1                                            )                                                                                            )                    ⁢                      (                                                            SyA                                                                              SyB                                                      )                                              Equation        ⁢                                  ⁢                  (          1          )                    where R1(t) represents the receive signal of antenna 905 in FIG. 9.
As will be understood from this equation, the STBC transmission scheme transmits data symbols SyA and SyB repeatedly at times t=i and t=i+1.
With the non-STBC scheme, on the other hand, the following equation holds:
                              (                                                                      R                  ⁢                                                                          ⁢                  1                  ⁢                                      (                    i                    )                                                                                                                        R                  ⁢                                                                          ⁢                  2                  ⁢                                      (                    i                    )                                                                                )                =                              (                                                                                h                    ⁢                                                                                  ⁢                    11                    ⁢                                          (                      i                      )                                                                                                            h                    ⁢                                                                                  ⁢                    12                    ⁢                                          (                      i                      )                                                                                                                                        h                    ⁢                                                                                  ⁢                    21                    ⁢                                          (                      i                      )                                                                                                            h                    ⁢                                                                                  ⁢                    22                    ⁢                                          (                      i                      )                                                                                            )                    ⁢                      (                                                            SyA                                                                              SyB                                                      )                                              Equation        ⁢                                  ⁢                  (          2          )                    where R1(t), R2(t) represent the receive signals of the antennas 905, 906 shown in FIG. 9.
As will be understood from this equation, the non-STBC transmission scheme transmits data symbols SyA and SyB only at time t=1.
Thus, although the STBC scheme has a transmission rate inferior to that of the non-STBC scheme, it can be said to be better in terms of reception quality. Conversely, although the non-STBC scheme has a higher transmission rate, it is inferior in terms of reception quality.
FIG. 5 is a block diagram illustrating a transmitter in the base station device 901 of a communication system having a plurality of antennas.
In FIG. 5, a frame-generation instruction unit 506 decides the transmission method (STBC scheme or non-STBC scheme) based upon transmission-method request information transmitted from a terminal device 902, and instructs a data sequence generating unit 501 of the decided content by a frame-generating instruction signal S1.
In accordance with the instruction from the frame-generation instruction unit 506, the data sequence generating unit 501 generates a transmit digital signal S2 of a modulating signal A and a transmit digital signal S3 of a modulating signal B from the transmit data.
A modulator 502 is capable of performing modulation using a plurality of modulation schemes and modulates the transmit digital signal S2 of modulating signal A, which has been output from the data sequence generating unit 501, using the modulation scheme of which it has been instructed. The modulated signal is converted to a radio-frequency signal by a wireless unit 503 and the modulated signal is transmitted from antenna 903.
Similarly, a modulator 504 is capable of performing modulation using a plurality of modulation schemes and modulates the transmit digital signal S3 of modulating signal B, which has been output from the data sequence generating unit 501, using the modulation scheme of which it has been instructed. The modulated signal is converted to a radio-frequency signal by a wireless unit 505 and the modulated signal is transmitted from antenna 904.
FIG. 6 is a block diagram illustrating a receiver in the terminal device 902 of the communication system having a plurality of antennas.
In FIG. 6, antenna 905 receives a combined signal that is the result of combining signals transmitted from the antennas 903 and 904 of the base station device 901, and a wireless unit 601 converts this receive signal to a desired frequency and supplies the resultant signal to a despreading unit 602. The latter despreads the receive signal and supplies the despread signal to a first transmission channel estimating unit 603, a second transmission channel estimating unit 604 and a data demodulator 607.
In accordance with a synchronizing signal generated by as frame synchronizer (not shown), the first transmission channel estimating unit 603, using a pilot signal, estimates the transmission channel of modulating signal A in the signal that has been output from the despreading unit 602. The estimated transmission-channel information of modulating signal A is output from the first transmission channel estimating unit 603 to the data demodulator 607 and to an eigenvalue calculating unit 609. The transmission-channel estimation signal of modulating signal A corresponds to h11(t) in Equation (2).
In accordance with a synchronizing signal generated by as frame synchronizer (not shown), the second transmission channel estimating unit 604, using a pilot signal, estimates the transmission channel of modulating signal B in the signal that has been output from the despreading unit 602. The estimated transmission-channel information of modulating signal B is output from the second transmission channel estimating unit 604 to the data demodulator 607 and to an eigenvalue calculating unit 609. The transmission-channel estimation signal of modulating signal B corresponds to h12(t) in Equation (2).
It should be noted that since the wireless signal received by antenna 906 is subjected to similar processing in a wireless unit 611, despreading unit 612, first transmission channel estimating unit 605 and second transmission channel estimating unit 606, this processing need not be described. A transmission-channel estimation signal that is output from the first transmission channel estimating unit 605 to the data demodulator 607 corresponds to h21(t), and a transmission-channel estimation signal that is output from the second transmission channel estimating unit 606 to the data demodulator 607 corresponds to h22(t).
In accordance with a timing signal that has been output from a frame synchronizer (not shown), the data demodulator 607 demodulates the signals, which have been output from the despreading units 602 and 612, using the output signals from the first transmission channel estimating units 603, 605 and the second transmission channel estimating units 604, 606. As a result, the receive digital signal of modulating signal A and the receive digital signal of the modulating signal B are obtained. At this time the data demodulator 607 identifies the signal transmission method (STBC scheme or non-STBC scheme) from transmission-method notification symbols contained in the pilot signals of the signals that have been output from the despreading units 602 and 612 and demodulates the data symbols in accordance with the content of the identification.
A reception field-strength detector 608 detects the reception field strengths based upon the signals that have been output from the despreading units 602, 612 and outputs the results of detection to a transmission method/modulation format decision unit 610.
When the transmission-channel information that has been output from the first transmission channel estimating units 603, 605 and the second transmission channel estimating units 604, 606 have been put into the form of a channel matrix, as indicated by Equation (2), the eigenvalue calculating unit 609 calculates the eigenvalue thereof and outputs the value to the transmission method/modulation format decision unit 610.
On the basis of the eigenvalue signal that has been output from the eigenvalue calculating unit 609 and the detection signal that has been output from the reception field-strength detector 608, the transmission method/modulation format decision unit 610 decides the transmission method and modulation format which the base station device 901 applies at the start of communication. Specifically, the transmission method/modulation format decision unit 610 decides upon either the STBC scheme or non-STBC scheme as the transmission method and decides QPSK, QAM, etc., as the modulation format.
The information decided is transmitted from the terminal device 902 to the base station device 901 as the transmission-method request information, and the base station device 901 performs transmission by the transmission method that is based upon this information.
Conventionally, in such wireless communication that utilizes the STBC scheme and the non-STBC scheme, which of the two transmission schemes should be selected and changed over to is performed based upon the reception field strengths of the receiver, as in the example of the prior art described above.
Further, the selection and changeover is performed also in accordance with quality of service (QoS) and reception SNR (signal-to-noise ratio) of the transmit data and the number of communicating parties.
However, which of the two transmission schemes is suited to high-speed transmission cannot be uniquely decided from reception field strength and reception SNR. One reason for this is that the optimum reception SNR varies greatly depending upon the modulation scheme (number of multiple values) used in communication.